The present invention relates to core functionalized star block copolymers and processes for making them. More particularly, the invention relates to novel star block copolymers which have asymmetric bi- or polyfunctional capping agents extending from the cores of the block copolymer.
Star block copolymers of conjugated dienes and/or vinyl aromatic hydrocarbons are well known and have been made commercially for many years. Such block copolymers are usually made by anionic polymerization of the monomers to form living polymer arms which are then coupled with a polyfunctional coupling agent which usually is a polyalkenyl aromatic compound such as divinyl benzene. After the coupling of the arms to the core is completed, the core is still "living" and capable of further reaction. The living core is terminated by the addition of alcohols, water, acids, or other acidic (protic) species.
For many reasons, it would be advantageous to introduce polar or low surface energy functionality into such star polymers. For example, polar functionality could increase the adhesion to polar surfaces of these polymers and lead to their wider use in adhesives or release agents. Such polymers might also be used in asphalt modification.
U.S. Pat. No. 4,417,029 describes star block copolymers having functional groups associated with the nucleus of the copolymers. Most of the derivatizing agents described in this patent are monofunctional compounds. In every case, this monofunctional group is changed by the derivitization reaction itself, often times to something less desirable. The few examples given of bifunctional derivatizing agents are symmetric, i.e., all of the functional groups are identical. In such a case, undesirable crosslinking tends to occur since the reaction is purely a statistical reaction. Crosslinking can be avoided by using a large excess of the derivitizing agent but this creates the disadvantage of having to remove a large amount of unreacted excess derivatizing agent,
The present invention provides a method for avoiding these disadvantages. Since the capping agent contains the functionality which is desired to extend from the polymer core and also a functional group of high reactivity for the capping reaction and different from the desired functional group, high selectivity to the reaction which allows the desired functional group to extend from the core is obtained. This is obtained without altering the chemical structure of the desired functional group and without undesirable crosslinking reactions. Furthermore, this invention allows the incorporation of functionality which was previously impossible to incorporate in anionic polymers, i.e. fluorine, NR.sub.2, (OCH.sub.2 CH.sub.2).sub.x OR, and metallocene, since they are unreactive toward the lithiated core or are destroyed by it through metallation (lithiation).